Molecular mechanisms of bacterial toxins Flashcards
function of bacterial toxins?
to cause damage to cells, tissues or the whole host organism, thereby contributing to disease
- undermine structure
- damaging and blocking immunty
Promote survival or spread of bacteria - hyaluronidase, collagenases
Damage or destroy cells/cell membranes - phospholipases; pores
Interfere with cell metabolism - cholera, diphtheria
Affect nerves - neurotoxins - botulism and tetanus
exotoxin
released proteins found in Gram+ve and Gram -ve
endotoxin
- powerful immunostimulants
- portion of LPS found in G-ve cell walls
- (also lipotheicoic acids of Gram +ve)
examples of toxins?
examples: cytotoxins enterotoxins neurotoxins leukocidins ciliostatic toxins
General Structure of Lipopolysaccharide endotoxin
polysaccharide and lipid
polysaccharide has an o-specific side chain, outer core and inner core
lipid is the lipid A part
essential functions of LPS?
Permeability barrier - allows only low molecular weight, hydrophilic molecules. Prevents penetration by bile salts and other toxic molecules; barrier to lysozyme and many antimicrobial agents.
Impedes destruction of the bacteria by serum components and phagocytic cells. – e.g. complement; macrophages
Role as an adhesin used in colonization of the host.
Both Lipid A (toxic component) and the polysaccharide side chains (the nontoxic but immunogenic portion) act as determinants of virulence in Gram-negative bacteria.
functions of variations in LPS structure
provide for the existence of different antigenic strains of a pathogen that may be able to bypass a previous immunological response to a related strain – immune evasion
Actions of Endotoxin
Activation of macrophage/monocyte cells
Macrophage/monocytes release IL-1, IL-6, IL-8, platelet activating factor and tumour necrosis factor (TNF-alpha). These also stimulate production of prostaglandins and leukotrienes.
Cytokines act at various sites including endothelium, liver, clotting cascade.
- activates complement via alternative pathway - can directly activate clotting cascade - cause polyclonal expansion of B-cells and secretion of immunoglobulins - increase phagocytic activity of macrophages
SEPSIS
increased vascular permeability, hypotension leading to hypovolaemic shock, fever, disseminated intravascular coagulation (DIC), multiple organ failure
PAMPs and PRRs - example
e.g. LPS and TLR4
Links innate immunity recognition to adaptive immunity via signalling
Types of Toxins
Classified by site of action
Type I - at cell membrane - not transported in
Type II - on cell membrane - membrane damage
Type III - intracellular effect after translocation
Extracellular - cellular matrix or connective tissue
signalling process of LPS
Here is LPS and how it causes production of pro-inflammatory cytokines
CD14 is a host protein that recognises and binds LPS
Then combines with MD2 protein and binds TLR4
Type 1 toxin?
Stimulates signalling proteins, Membrane Guanyl cyclase (GC)
- changes intracellular cGMP
e. g. E.coli ST enterotoxin. Once it is on food, it can’t be removed as it’s temp stable. Binds G protein coupled receptor, increases cGMP, less regulation of electrolytes, Leads to Travellers diarrhoea. Temperature stable (ST) peptides
e. g. Streptococcus pyogenes, erythrogenic toxin - Scarlet fever
-Lysogenic conversion - phages
-Pyogenic Exotoxin types A, B, and C
(SPE-A, SPE-B, SPE-C)
-Toxin locates to skin damages membranes
of capilliaries
-punctate erythematous rash
(blanches - oedema filled haemolysis)
-tongue - raw - strawberry papillae
-desquamation
-complications - cellulitis + septiceamia
Toxic Shock Syndrome Toxin (TSST-1)
- 22 kilodalton peptide controlled by agr-quorum sensing
- Located on a pathogenicity island.
Causes Toxic Shock Syndrome whose symptoms include: fever, desquamative skin rash, hypotension,
multiple system involvement, and potentially death.
Organism carrying the toxin is usually phage type II.
Absorbed toxin induces production of IL-1B and TNF by monocytes.
Superantigens
The superantigens recognise the variable region of the TCR of a subset of T cells, not every superantigen will recognize every TCR variable region, but a subset, they have some specificity. Some superantigen will elicit proliferation of subset of T cells
-activate subsets of T lymphocytes to liberate cytokines
-mediates major systemic effects:-
fever, hypotension, skin lesions, shock,
multi-organ failure and death
-binds with high affinity to MHC class II receptors monocytes and macrophages
- distinct from the classic antigen binding groove
- Recognized by variable region, Vb, of the T-cell receptor of subsets of T lymphocytes.
Approaches to treating Toxic Shock Syndromes
- deal with the shock
giving fluids, bring the blood pressure back up - get rid of whatever’s producing the superantigen
- usually this means appropriate cocktail of antibiotics
- dampen down the over-active immune response
- steroids
given an example of an Exfoliative Exotoxin
Extracellular
Staphylococcus scalded skin syndrome (SSSS) or Ritter’s Disease (s aureus)
This toxin works at the surface, targets a protein in the desmosomes that’s imp in holding skin layers together - skin layers separate as a result of toxin, allows bacteria to invade and divide
2 types:
1 - ET-A is chromosomally encoded.
2- ET-B is plasmid encoded. Made by 5% of S. aureus strains.
- Cleaves stratum granulosum layer by splitting desmosomes.
- No inflammatory response is seen.
- Targets Desmoglein-1a cadherin found only in superficial epidermis (zona granulosa).
- Phage group II.
- Toxin is antigenic, and circulating antibody confers immunity.
Type II toxin?
On cell membrane
-phospholipase C, pore formation, enzymic disruption and haemolysins
Thiol-activated haemolysins
- Binds cholesterol,
- oligomerises – heptamer pore - Damages cellular membranes/matrices
- Disrupts ion transport – cell lysis
name some bacteria and their toxins
Strep.pyogenes streptolysin
Strep.pneumoniae pneumolysin
Listeria monocytogenes listeriolysin (intracellular)
Bacillus cereus cerolysin
Clostridium perfringens perfringolysin
Staphylococcus aureus α-toxin
what bacteria causes gangrene?
Clostridium perfringens
describe clostridium perfringens
Gram +ve rod
Anaerobe
Spore forming
Soil and human gut
Virulence factor- α-lecithinases
Phospholipase C
– lipid membrane damage
Kills RBCs, WBCs
If escapes into blood stream
– severe haemolysis and death
Type III toxin?
Type III - intracellular action after translocation of active sub-unit
Classified by enzymatic action
- ADP-ribosylation cholera; diphtheria;pertussis
- N-glycosidase Shigella; EHEC O157:H7
- Glucosyl transferase C.difficile (G proteins)
- Zn2+ endopeptidase Botulism; tetanus
Classified by molecular target or effect:-
ADP ribosylation is a way that you can control post translational modification – this will dysregulate a target
N-glycosidases – will break down specific parts of RNA to stop protein synthesis
Glucosyl transferase will modify different G-proteins inside a cell
Molecular structure of AB5 bacterial toxins
5x binding domains
active, enzymatic components
-Diphtheria toxin molecular structure
Toxins that inhibit protein synthesis
Shigella - Dysentery
Shiga toxin - AB5 - (active A subunit and 5 binding subunits B)
N-glycosidase activity
globotriasylceramide (Gb3) receptor
Cleaves adenosine from 28S rRNA
C.diphtheriae - Diphtheria
- toxin A-B type
- toxin-receptor internalisation and cleavage to release active A sub-unit
ADP-ribosylates EF-2
Diphtheria
Pharynx - non-invasive multiplication
Toxin produced locally but acts at a distance
- absorbed by lymphatics – systemic effects
fever, pallor, weakness
polyneuritis
myocarditis ( ~2 weeks)
damages heart, kidney, nerves, adrenals
- kills epithelial cells and polymorphs
gelatinous exudate
ulcer - necrotic exudate - Pseudomembrane- local inflammation ,swelling, lymph nodes - Bull Neck
- Respiratory obstruction
Some bacterial toxins target second messenger signalling molecules
Guanyl cylcase receptor
- E.coli heat-stable (ST) enterotoxins binds GC receptor
- ↑cGMP
small Rho GTP binding proteins
-Regulate actin cytoskeleton
-inactivated by:-
Clostridium botulinum C3 toxin - ADP-ribosylation
Clostridium difficile toxins A and B (CdA, CdB)
- glucosylation
-activated by deamination with
E.coli cytotoxic necrotizing factor (CNF)
Bordetalla pertussis dermonecrotic toxin (DNT)
G Proteins - Targets for bacterial toxins
Pertussis toxin ADP-ribosylates α-Gi-GDP Blocks dissociation no Gi activity ↑cAMP
Cholera toxin ADP-ribosylates Gs- α locks in active form continous activity ↑cAMP
E.coli : friend or foe?
Commensal – gut organism Pathogen – diarrhoea, dysentry Haemolytic uraemic syndrome (HUS) Urinary tract and kidney infections Septicaemia Pneumoniae and meningitis
Virulence factors – toxins, adhesins, invasins
(plasmids)
Whooping Cough: a toxin mediated disease
Invasive adenylate cyclase
lethal toxin (dermonecrotic toxin) - superantigen -
tracheal cytotoxin
pertussis toxin, PTx
Net effect -
permits multiplication at mucosal surface
prevents localised immune activation and attack
promotes survival and transmission
Toxins Produced by Bordetella pertussis
adenylate cyclase (acts locally) - inhibits chemotaxis, phagocytosis, neutophils, ↑permeability
lethal toxin - inflammation, vasoconstriction
tracheal cytotoxin (TCT) kills ciliated cells + neutrophils acts early, induces IL-1
pertussis toxin, PTx - attachment to cilia
- effects cAMP, phospholipases, ion channels via
- multiple G protein interference (Gi, GPL,Go, GK)
- blocks migration of Mps and neutrophils;
- blocks nerve impulses, muscle contraction
Net effect - permits multiplication at mucosal surface
prevents localised immune activation and attack and promotes survival and transmission
Overcome Host Defences - Muco-ciliary clearance mechanism
Release of cytotoxin
Specific damage
Attachment
Molecular Pathogenesis of Whooping Cough
diagram
Dual Function of B subunit of Pertussis Toxin, PTx
- Binding and transmembrane transport of active A sub-unit of PTx
- Direct mitogenic activity triggers cascade of non-cyclic mediators
- phospholipases, phophatidyl inositol,
- arachidonate release - PGE1
→ lymphocytosis; ↑ vascular permeability; haemaglutinin activity
Pneumolysin of Streptococcus pneumoniae
This is a multi functional toxin promotes pathogenicity
- Cytolytic - pore former
- impairs complement mediated phagocytosis?
- inhibits the migration and anti-bacterial functions of
PMNs and macrophages
- blocks proliferative response of, and IgG production by lymphocytes
- induces TNF, IL-1B inflammation and tissue damage
- inhibits cilia beating in respiratory tract
- specific cochlear damage in animal models
- Pneumolysin mutants – reduced virulence, cleared rapidly from blood stream and no cochlear damage
Body site targets for Bacterial Toxins
Intestinal Epithelium - Cholera toxin, Shigella toxin and E.coli enterotoxin
Connective Tissue - Clostridium perfringens α toxin, Streptococci, Collagenase, Hyaluronidase
Blood, capilliary - Staph. aureus - leucocidins
& defence cells - TSST
Skin - Staphs. epidermolytic toxin, Strep. pyogenes and erythrogenic toxin
CNS - tetanus
Neuromuscular junction - botulinum toxin
